National trends in rheumatoid arthritis and osteoarthritis prevalence in South Korea, 1998–2021

Studies on the trends in the prevalence of rheumatoid arthritis (RA) and osteoarthritis (OA) are limited, particularly during the COVID-19 pandemic. This study aimed to analyze the temporal trend of RA and OA in South Korean adults from 1998 to 2021, including the COVID-19 pandemic period. The Korea National Health and Nutrition Examination Survey (KNHANES) data on adults aged ≥ 19 years were analyzed to investigate the prevalence of RA and OA from 1998 to 2021. The prevalence trends were compared by the years, and βdiff (β difference) was calculated. Odds ratios (ORs) were computed for each disease to examine changes in disease prevalence before and during the pandemic in order to determine the impact of the pandemic on disease prevalence. Among 163,221 Korean adults, the prevalence of RA and OA showed a steady decrease from 2005 (RA: from 1.91% in 2005–2007 to 1.55% in 2016–2019 and OA: from 9.75% in 2005–2007 to 8.27% in 2016–2019), but there was a slight increased after the onset of the COVID-19 pandemic (RA: from 1.23% in 2020 to 1.36% in 2021 and OA: from 8.04% in 2020 to 8.27% in 2021). Vulnerable groups, including participants aged ≥ 60 years (versus 19–60 years, ratio of ORs: 1.222; 95% CI 1.011–1.477), urban residents (ratio of ORs: 1.289; 95% CI 1.007–1.650), and participants with higher education level (ratio of ORs: 1.360; 95% CI 1.119–1.653) showed higher ORs of OA, whereas no particularly vulnerable population was observed for RA. Our findings provide an insight into the long-term trends of RA and OA among adult population and highlight a novel perspective on the impact of COVID-19 on disease prevalence.


Ascertainment of RA and OA
The objective of our study was to investigate the risk factors related to the two most common types of arthritis, RA and OA, over a period of 24 years from 1998 to 2021.To achieve our research objective, we surveyed a large sample of participants and asked them the question: "Have you ever been diagnosed with RA or OA by a doctor?"Based on their answers, we categorized the participants into three groups: RA, OA, and both 13 .We collected data on various potential risk factors associated with the development of RA and OA, such as age, sex, lifestyle habits, and socioeconomic status.We conducted statistical analyses to examine the associations between these risk factors and the development of RA and OA and to identify any patterns or trends that emerged over 24 years.

Statistical analyses
The results of this study were presented using qualitative data expressed as proportions or percentages.Weighted multivariate regression model analyses were conducted to compare the estimates of each related factor before and during the COVID-19 pandemic, with weighted odds ratios (ORs) with 95% confidence intervals (CIs) 19 .The prevalence of RA and OA was calculated using data from the KNHANES, spanning from 1998 to 2021, stratified by year group.Weighted complex sampling analysis was performed to ensure accurate estimation.Binomial or linear logistic regression models were used to compute the ORs with 95% CIs or β-coefficients with 95% CIs.To ensure robustness of the main findings, a stratification analysis was performed using variables such as sex, educational level, region of residence, and income in all the regression models.Furthermore, the ratio of ORs was calculated to estimate the interaction term of each risk factor and identify groups that were more vulnerable to the patient with RA and OA during the pandemic.Overall, this study aimed to provide a comprehensive analysis of the impact of the COVID-19 pandemic on the prevalence of RA and OA and to identify the factors that contribute to vulnerability to these conditions.The SAS software (version 9.4; SAS Institute, Cary, NC, USA) was used for statistical analyses, with a two-sided test, and a p-value ≤ 0.05 was considered statistically significant 19 . www.nature.com/scientificreports/

Results
We recruited 163,221 participants from 1998 to 2021 with the following distribution of characteristics: age (19-29 years, 18  1. Table 2 illustrate the prevalence of RA, OA between pre-pandemic and during the pandemic.The prevalence of RA and OA showed a U-curve between pre-pandemic and during the pandemic (RA: from 1.46% [95% CI 1.38   S1).
Table 3 and Fig. 1 show the national trends in the prevalence of overall RA and OA, as well as the prevalence of RA and OA separately, along with the β-coefficients of the ORs before and during the COVID-19 pandemic.A statistically significant decrease in the weighted prevalence of RA was observed pre-pandemic (2005-2007  S2).While investigating the weighted OR in 2021 with respect to that of 2020 (reference), there was a surge regarding RA among underweight group (OR, 6.62 [1.28 to 34.36]).
Table 4 shows the pandemic-related effects on vulnerable groups of patients with RA and OA.A statistically significant difference was observed among vulnerable individuals with OA as follows: participants ≥ 60 years old (ratio of ORs 1.222 [95% CI 1.011 to 1.477]), urban residents (ratio of ORs 1.289 [1.007 to 1.650]), and participants with a high level of education (ratio of ORs 1.360 [1.119 to 1.653]).In contrast, no significant difference in pandemic-related effects was observed among vulnerable individuals with RA (Table S3).

Key results
The present study examined trends in the prevalence of RA and OA over a 24-year period from 1998 to 2021 and evaluated the differences in prevalence before and during the COVID-19 pandemic (n = 163,221).The prevalence of RA and OA consistently declined before the onset of the pandemic, but there was a slight increased during the pandemic.However slight increasing prevalence during pandemic was not statistically significant.Notably, patients with OA had a significantly high prevalence among vulnerable groups, including individuals aged ≥ 60 years, urban residents, and those with a high education level.Therefore, these findings suggest policy researchers should develop personalized policy proposals to address the needs of these groups during the pandemic.

Global epidemiology and mechanism
Previous studies have reported a global increase in the prevalence of RA and OA, before the COVID-19 pandemic 20 .However, in South Korea, the prevalence of both conditions has consistently declined, contrary to global trends 21 .Similar trends of decline in the prevalence of RA have been observed in Japan and Sweden as well 22 .A Japanese study found a correlation between a decrease in RA prevalence and changes in lifestyle factors, such as dietary habits and smoking, while a Swedish study has linked this decrease to improvements in medical accessibility for patients with RA 16,23 .Although we cannot accurately identify the specific reasons for the variation in RA prevalence across countries, we believe that factors, such as genetic, cultural disparities, and health level may have a significant impact 24 .In addition, increased BMI and muscle weakness are closely related to OA and RA, as these diseases are associated with factors that contribute to joint strain 25,26 .
During the COVID-19 pandemic, the social distancing policies and the closure of sports facilities significantly diminished the public health standards, which could generally explain the observed increase in the prevalence of RA and OA 27,28 .
RA is a complex disease with multiple factors contributing to its development, with genetics potentially being a significant factor 29 .Other studies have reported that numerous variables can affect the prevalence of RA 30 .Therefore, further research is warranted to investigate this matter thoroughly.An increase in the prevalence of OA and RA was the most prominent in the highly educated older adult population living in urban areas 31 .These individuals who were retired and had reduced health standards due to COVID-19 were most affected 32 .According to previous research, the domestic standard of physical activity among South Korean adult population exhibited a declining trend until 2017, followed by an upturn until 2019 33 .In 2020, confronting the pandemic, there was a significant decline in physical activity.This pattern closely aligns with the prevalence trends in RA and OA, which could substantiate our hypothesis.
This finding suggests that OA may occur when physical activity falls below a certain threshold, which warrants further investigation.It is crucial to recognize this issue at a national level and implement policies to encourage exercise among the older adults, prevent the occurrence of OA, and provide the appropriate treatment for those with the disease.www.nature.com/scientificreports/

Strengths and limitations
The present study is significant in utilizing population-based nationwide representative data to compare the trends and prevalence of RA and OA, both before and during the COVID-19 pandemic 34 .The use of national data is particularly noteworthy, as individual data collection during a pandemic could have been challenging.Furthermore, this study's utilization of survey data collected from 1998 to 2021, spanning a period of 24 years, adds academic value to the research findings due to its long-term nature.However, this study has several limitations.First, data were collected only from Koreans, thus limiting generalizability of the results to other ethnic groups or countries.Further research with multiple ethnicities and different countries is necessary to examine the global impact of the COVID-19 pandemic 35 .Second, data on patients with separate covariate of RA and OA were only available from 2005-2021and information prior to 2005 was not available.Third, some variables in the study, such as height and weight, were self-reported, which may have introduced bias.To address this concern, the KDCA should include a question in the survey about when participants' heights and weights were last measured 36 .In addition, the diagnosis of RA and OA was self-reported, potentially resulting in a recall bias.Lastly, we substituted physical activity data from the previous study, since we were unable to conduct our own analysis of domestic physical activity levels.

Conclusion
Our study identified long-term trends in the prevalence of RA and OA spanning 24-years from 1998 to 2021, especially focusing on the COVID-19 pandemic.The results showed a consistent decline in the prevalence of both RA and OA until 2016 followed by an oscillation until 2021, where significant drop occurred in 2019.Notably, OA exhibited a higher prevalence among vulnerable groups, such as individuals over 60 years of age, urban residents, and those with a high education level.When investigating the RA prevalence trend, there was no specific vulnerable population.This outcome will help government policy researchers devise personalized healthcare policies targeting the vulnerable groups.While our study did not find any evidence of a relationship between the COVID-19 pandemic and the prevalence of RA, additional follow-up studies is needed to further explore this topic.

Table 3 .
National trend of the RA and OA prevalence and β-coefficients of the odds ratios before and during the COVID-19 pandemic, weighted % (95% CI), in the data obtained from the KNHANES.The numbers in bold indicate a significant difference (p < 0.05).BMI body mass index, CI confidence interval, KNHANES Korea National Health and Nutrition Examination Survey, OA osteoarthritis, OR odds ratio, RA rheumatoid arthritis.

Figure 1 .
Figure 1.Seventeen-year trends in the prevalence of rheumatoid arthritis and osteoarthritis in South Korea, 2005-2021.OA osteoarthritis, RA rheumatoid arthritis.

Table 4 .
Difference between pre-and during the COVID-19 pandemic by the ratio of ORs on OA and RA, weighted % (95% CI), in the data obtained from the KNHANES.The numbers in bold indicate a significant difference (p < 0.05).KNHANES Korea National Health and Nutrition Examination Survey, CI confidence interval, OA osteoarthritis, OR odds ratio, RA rheumatoid arthritis.

Table 2 .
National trends of the prevalence of RA and OA before and during the COVID-19 pandemic, weighted % (95% CI), in the data obtained from the KNHANES.CI confidence interval, KNHANES Korea National Health and Nutrition Examination Survey, OA osteoarthritis, OR odds ratio, RA rheumatoid arthritis.*Prevalence of RA, OA were available from 2005.